1. Field of the invention
The present invention relates to novel macromonomers useful in the preparation of biomedical devices. More particularly, this invention relates to siloxane macromonomers which can be copolymerized with hydrophilic or hydrophobic monomers to prepare biomedical devices such as contact lenses. This invention includes the macromonomers, intermediate polymers used to prepare the macromonomers and the copolymers prepared from the macromonomers.
2. Background of the Invention
Various monomers useful in the preparation of biomedical devices are known. For example, contact lenses, one type of biomedical device, consist of polymers and copolymers having various types and concentrations of monomers. Rigid gas-permeable (RGP) contact lenses generally include silicone-containing monomers which are generally hydrophobic in nature. Soft contact lenses or "hydrogel" lenses are generally made from hydrophilic monomers such as poly-hydroxyethyl methacrylate.
Until recent years, RGP contact lenses and hydrogel contact lenses had distinct properties. Hydrogel contact lenses, particularly those with high water content, tended to be weak, subject to deposit formation, and exhibit lower visual acuity than RGP lenses. RGP contact lenses tended to be less comfortable, had low water content, and generally exhibit higher visual activity than hydrogel or "soft" lenses. RGP lenses often required wetting additives or surface coatings to provide adequate wettability.
More recently, the distinction between RGP contact lenses and hydrogel contact lenses has become blurred. For example, hydrophilic monomers have been copolymerized with certain siloxane monomers, such as 3-methacryloxypropyltris (trimethyl siloxy) silane (hereinafter TRIS), and dimers to produce contact lenses. For example, see U.S. Pat. Nos. 4,711,943 and 4,182,822 which discuss such copolymers.
New polymeric materials are continually being developed for biomedical devices which exhibit the strength and durability normally found with silicone-containing polymers yet provide the flexibility and water content often found with hydrophilic polymers. In the case of contact lenses, it is desirable to combine the beneficial properties of such polymers to obtain a copolymer having the best possible physical properties for overall lens performance and comfort. Thus, it continues to be desirable to obtain a contact lens having enough resilience in the hydrated state to maintain adequate lens shape. At the same time, the lens should not be so rigid as to cause any physiological problems such as corneal staining or flattening of the cornea, or other discomfort. On the other hand, the lens should not be too soft, or it will be difficult to handle and will tear easily during normal use. Further, if the lens is too soft, it will often result in poor optics.
It should also be noted that many additional factors can affect the performance of the copolymer materials. For example, the type and concentration of crosslinkers employed, which generally tie together long polymer chains, can significantly vary the properties of the materials. Further, various modifying agents can be employed to achieve desired physical visco-elastic and/or physiological properties such as strength, modulus, and various surface properties.
As described above, the use of TRIS monomers in RGP lenses is well known. TRIS is typically used as a strengthening agent or a modulus modifier and aids to increase oxygen permeability. Generally, TRIS lowers the modulus of the final copolymer. However, the present macromonomer can be advantageously employed to increase the silicone content of the copolymer and thereby increase the oxygen permeability without affecting the modulus of the copolymer to the extent experienced with TRIS. In fact, the present siloxane macromonomer can be employed in hydrogel copolymers in amounts of up to about 90 percent by weight of the final copolymer and still exhibit typical hydrogel characteristics.
The present invention is directed to novel siloxane macromonomers which can be copolymerized with hydrophilic monomers to prepare novel polymeric materials useful as biomedical devices and particularly useful as contact lens materials. Further, these macromonomers can be randomly copolymerized to produce elastomers having high oxygen permeability. Even further, these macromonomers can be randomly copolymerized with other classes of monomers, including those typically used as crosslinking agents and strengthening agents, to prepare polymeric materials which are strong, durable, and have high oxygen permeability.